A structurally simple fluorescent receptor (receptor 1) has been synthesized and was found to show a dramatic enhancement in its fluorescence emission upon complexation with Mg2+. This was maybe contributed to by the inhibition of the CN isomerization in the excited state. The experimental results show that the receptor was selective and sensitive towards Mg2+ in the presence of competing ions, with a low detection limit of 3.5 × 10−9 mol L−1 (3σ).